Spread-spectrum techniques have been proposed for reducing the input switching harmonics in power supplies in order to meet the IEC electromagnetic compatibility (EMC) requirements.
The constant-switching actions of power converters generate harmonics at fixed frequencies as shown in FIG. 1. IEC regulations impose limits on the current harmonic components on the input side. The harmonics generated by the constant-switching power converter cause “conducted” electromagnetic interference (EMI) because the current harmonics can adversely affect all other instruments using the same power supply through the “conducting” cables.
In particular, if the power converter switches at a constant frequency A, the input current harmonic power spectrum typically consists of discrete harmonic components B, as shown in FIG. 1. The existence of these discrete harmonics means that the power of the harmonics (which can be considered as noise) concentrates at a few discrete frequencies in the spectrum. Since the power concentrates at a few discrete frequencies, the magnitude of the harmonic power is considerable. Therefore, spread-spectrum techniques have been proposed to reduce the magnitude of the input current harmonic power for switching power converters.
FIG. 2 shows a schematic diagram of a spread-spectrum power converter and the typical input current harmonic spectrum exhibited by the converter. Spread-spectrum techniques vary the switching actions in different ways so as to vary at least one switching parameter (such as switching frequency, pulse position, etc) in order to generate the harmonics in slightly different ways. Thus, the switching harmonics do not remain at fixed frequencies all the time. Consequently, the average harmonic power can be spread over a wide range of the spectrum with a significantly reduced magnitude, as shown in FIG. 2.
In the prior spread-spectrum techniques described above, the purpose is to reduce the conducted EMI generated by power converters so that the quality of the power system is not deteriorated.